Cathode for duoplasmatron-type ion source

ABSTRACT

A central spike cathode is added to and is axially positioned through the conventional spiral filament-type cathode of a duoplasmatron-type ion source such that the current output and cathode lifetime thereof are substantially increased. The added spike cathode is so positioned that it is the closest item to the intermediate electrode of the ion source. Further increase in operating performance can be obtained when a cylindrical shield is utilized to surround the cathode assembly. The spiral filament may be turned off after initiating the arc of the ion source.

United States Patent inventor Robert RJhI 2,007,923 Concord, Tenn. 2,231,587 Appl. No. 871,889 2,249,672 Filed Oct. 28, 1969 2,559,227 Patented Aug. 31, 1971 3,340,425 Assignee The United States'ot America as represented by the United Stats Atomic Enery Conunhsion CATHODl-I FOR DUOPIASMAI'RON-TYPE ION SOURCE 3 Claims, 3 Drawing Figs.

US. (I 3 13/309, 313/338, 313/342, 313/351 Int. CL H01] 1/52 Field Search 3131309, 338, 342, 351,232,332;315/1li Reference Cited UNITED STATES PATENTS 1,992,071 2/1935 l-leaiey 313/309 X Braselton 313/338 X Miles 313/332 X Spanner 313/351 X Rieber.... 313/232 X Kelley 315/1 11 Primary Examiner- David Schonberg Assistant Examiner-Paul A. Sacher Attorney-Roland A. Anderson ABSTRACT: A central spike cathode is added to and is axially positioned through the conventional spiral filament-type cathode of a duoplasmatron-type ion source such that the current output and cathode lifetime thereof are substantially increased. The added spike cathode is so positioned that it is the closest item to the intermediate electrode of the ion source. Further increase in operating performance can be obtained when a cylindrical shield is utilized to surround the cathode assembly. The spiral filament may be turned off after initiating the arc of the ion source.

PATENTEU AUG3I I97! .53 mormm Elm INVENTOR? Robert R. Ha II BY m ATTORNEY.

CATHODE FOR DUOPLASMATRON-TYPE ION SOURCE BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contact with the U.S. Atomic Energy Commission.

The field of art to which the present invention relates is for ion sources. Duoplasmatron-type ion sources are utilized in several research programs at various government laboratories. For example, such ion sources are used for the injection of ions into various of the controlled thermonuclear reactor devices and for various accelerator systems. The ion source of the present invention is an improvement over the ion source described in U.S. Pat. No. 3,340,425, issued Sept. 5, 1967, to George G. Kelley.

In some applications of prior ion sources in controlled thermonuclear research it has been desirable to obtain arc currents (between cathode and anode) of 50 of more amperes in order to provide large ion currents. The conventional-filaments of the prior ion source have a lifetime of only a few hours at an output of 50 amperes. Furthermore, the filament current required for adequate emission increases from about 18-20 amperes to about 28 amperes during the lifetime. The are voltage required to restart a discharge also increases with time and frequent voltage breakdown between the filament support posts and the intermediate electrode occurs. Thus there exists a need for an improved cathode in an ion source such that the lifetime thereof can be substantially increased. The present invention was conceived to meet this need in a manner to be described hereinbelow.

SUMMARY OF THE INVENTION With a knowledge of the limitations of the prior art as discussed above, it is the object of the present invention to provide a duoplasmatron-type ion source with an improved cathode arrangement such that the operating lifetime and current output thereof are substantially increased.

The above object has been accomplished by providing the ion source of the above-mentioned patent with an additional spike cathode which is axially positioned through the prior spiral cathode filament and which extends well into the converging inner portion of the intermediate electrode, with the spike cathode being mounted from one of the support posts for the filament cathode. In addition, in a preferred embodiment of the present invention, insulating sleeves are mounted around the support rods for the cathodes and a cylindrical shield is provided to enclose the filament cathode and about half of the spike cathode.

The provision of the above additional cathode has provided an ion source which can be operated at substantially higher ion source are currents while at the same time substantially increasing the operating lifetime of the cathode structure.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view illustrating the structure and position of a filament-type cathode of the ion source of the above prior patent.

FIG. 2 is a cross-sectional view illustrating the structure and position of one embodiment of the improved cathode arrangement of the ion source of the present invention.

FIG. 3 is a partial cross-sectional view of another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring briefly to FIG. 1, it may be seen that the conventional prior art filament-type cathode I was spiral in structure and was mounted on the ends of a pair of support posts 2, 3 in the same manner as was done in the above prior patent. The cathode 1 was positioned at about the beginning of the converging inner portion of an intermediate electrode 4. This cathode was typically fabricated from 0.040-inch tantalum wire, wrapped with platinum gauze, and dipped in an emission-inducing coating. The anode, which is not shown in any of the figures, is spaced just beyond the electrode 4 in the same manner as in the above-mentioned prior patent, and the remaining portions of the ion source are the same as in the prior patent.

It should be understood that the two embodiments of the present invention, as shown in FIGS. 2 and 3, show only the improved cathode arrangementsand that each of these embodiments includes the additionalistructure necessary to provide a complete ion source in the same manner as is described in the above-mentioned prior patentt-towhich reference is made. 3*:-

Referring now to FIG. 2, a cathode structure-for improved operation combines a conventional spiral .filarnent- 5 with an axially positioned tantalum rod 6 mounted from one of the support posts 7, 9. This cathode rod 6 extends well into the converging inner portion of intermediate electrode 8. Typically, the rod 6 may be one-eighth inch in diameter and about 2 inches long. Except for the cathode rod 6, the rest of the structure illustrated in FIG. 2 is identical to that shown and described in the above-mentioned prior patent. As in the prior patent, the posts 7 and 9 in FIG. 2 pass through insulating bushings and'are connected to a DC power supply, not shown in FIG. 2, in the same manner as was done in the prior patent.

An ion source having a cathode of the structure of FIG. 2 was operated to produce an arc of 50 amperes. 'A current of about 20 amperes was passed through the spiral filament. The resultant emission in the presence of an ionizable gas and a voltage applied between the cathode and anode generated an arc in the ion source. The are heated the central rod to emission temperature in about 15 seconds. Thereafter, the filament current supply, not shown, was turned off. The lifetime of this cathode was of the order of 17 hours before breakdown between the support posts and the electrode 8 occurred, which lifetime was substantially longer than that which could be achieved with the ion source of the above-mentioned patent.

The embodiment of the present invention, which is illustrated in FIG. 3, is a preferred embodiment in that an even greater cathode lifetime can be achieved when this structure is utilized in an source such as disclosed in the above prior patent. The device of FIG. 3 is substantially the same structure as shown in FIG. 2 except for the addition of insulating sleeves 10, 11 around support posts 7', 9' and a cylindrical shield 12 mounted on the same support post as the rod 6'. The shield 12 encompasses part of the insulated posts 7', 9, the spiral cathode 5, and a portion of the spike cathode 6'. This shield 12 typically is formed from 0.015-inch tantalum sheet and is about three-fourths inch in diameter and about 1% inches long. Its spacing from the intermediate electrode 8' must be greater than the tip of the cathode 6'. The main purpose of the insulating sleeves 10, 11 encompassing the posts 7' and 9', respectively, is to prevent arcing between the encompassing shield 12 and the cathode supp rt posts 7 and 9'.

The embodiment of FIG. 3 has been utilized in an ion source such as disclosed in the above prior patent which provided a 50-ampere arc for 27 hours with no apparent deterioration in performance. The arc was intentionally stopped and restarted each hour during this run to check the integrity of i the spiral filament. An identical unit was operated for a test period of 3 hours at an arc current of I00 amperes without appreciable deterioration in performance.

Thus, an ion source, such as described in the above-mentioned patent when provided with the cathode arrangement of either FIG. 2 or FIG. 3 of the present invention, will not only have all of the advantages outlined in the prior patent, but also have the advantage of providing substantially increased operating lifetime for the improved cathode structure as well as increased current output therefrom.

It should be understood that an ion source utilizing the structure of either FIG. 2 or FIG. 3 may be operated without using the filament cathode 5 by initiating emission from the spike cathode using R.F. voltage and a gas that is easily ionized, e.g., argon. When emission temperature is reached, hydrogen or other desired ion-producing gas would be substituted for normal operation.

This invention has been described by way of illustration rather than by limitation and it should be apparent that it is equally applicable in fields other than those described.

What is claimed is:

1. In a duoplasmatron-type ion source including a pair of cathode support posts, a spirally wound filament cathode connected to said posts, an elongated tubular intermediate electrode having a'tapered, apertured end portion and enclosing said cathode with the cathode being in axial alignment with the aperture of the tapered portion of said intermediate electrode, a DC power supply connected to said cathode through said support posts, the improvement comprising an elongated rod cathode axially positioned within said filament cathode and extending well into the tapered converging inner portion of said intermediate electrode in spaced relation to the aperture thereof, and said rod cathode being electrically connected to one side of said DC power supply, whereby during operation of said ion source said filament cathode serves to create an initial discharge from said rod cathode which heats said rod cathode to an emission temperature such that said DC power supply to said filament cathode may be disconnected, wherein the operating life of said filament cathode and the current output of said ion source are substantially increased.

2. The ion source set forth in claim 1, wherein there are provided insulating sleeves around said cathode support posts, and a tubular shield is provided which encloses a portion of said insulated posts, said filament cathode and the upper portion of said rod cathode, whereby said operating life of said filament cathode and said current output of said ion source are further increased.

3. The ion source set forth in claim 2, wherein said rod cathode and said tubular shield are tantalum. 

1. In a duoplasmatron-type ion source including a pair of cathode support posts, a spirally wound filament cathode connected to said posts, an elongated tubular intermediate electrode having a tapered, apertured end portion and enclosing said cathode with the cathode being in axial alignment with the aperture of the tapered portion of said intermediate electrode, a DC power supply connected to said cathode through said support posts, the improvement comprising an elongated rod cathode axially positioned within said filament cathode and extending well into the tapered converging inner portion of said intermediate electrode in spaced relation to the aperture thereof, and said rod cathode being electrically connected to one side of said DC power supply, whereby during operation of said ion source said filament cathode serves to create an initial discharge from said rod cathode which heats said rod cathode to an emission temperature such that said DC power supply to said filament cathode may be disconnected, wherein the operating life of said filament cathode and the current output of said ion source are substantially increased.
 2. The ion source set forth in claim 1, wherein there are provided insulating sleeves around said cathode support posts, and a tubular shield is provided which encloses a portion of said insulated posts, said filament cathode and the upper portion of said rod cathode, whereby said operating life of said filament cathode and said current output of said ion source are further increased.
 3. The ion source set forth in claim 2, wherein said rod cathode and said tubular shield are tantalum. 